1. Field of the Invention
The present invention relates to compositions for obtaining cured products which have superior initial conductivity which does not deteriorate substantially under high temperature and high humidity conditions. The present invention also relates to conductive cured resinous products and methods for making them.
2. Description of the Prior Art
The incorporation of metals, metallic salts, and metal complexes into resins to impart stability to the resin and/or conductivity to the resin is well-known in the art. U.S. Pat. No. 3,360,589 discloses hardenable, unsaturated polyester moulding masses which are stabilized by copper complex compounds. Soluble cupric compounds, it is disclosed, further improve or exceed the stabilizing effect of phenolic gelling inhibitors such as hydroquinone, tert-butyl-pyrocatechol, p-benzoquinone, and other gelling inhibitors such as aromatic amines, amine salts, and quaternary phosphonium and arsonium salts. The soluble cupric compounds, it is taught, have the disadvantage of causing a greenish-brown to greenish-blue discoloration of the originally colorless polyester moulding masses. The use of colorless cuprous compounds, such as cuprous halides was found to be unsatisfactory because large amounts of solubilizing agents were needed to obtain a homogeneous composition. Complex compounds of cuprous chloride and/or cuprous bromide with neutral phosphoric acid esters were found to improve storage stability and reduce discoloration of the polyeser moulding masses. Use of the composition to obtain conductive cured products is not taught in U.S. Pat. No. 3,360,589. U.S. Pat. No. 3,474,464 discloses a process for preparing metal acetylacetonates which can be used to enhance the stability of chlorine-containing polymers such as polyvinyl chloride. The production of conductive cured products is not disclosed.
U.S. Pat. No. 3,647,532 discloses a conductive ink composition which contains copper powder, a reducing agent such as hydrazine hydrate, glass frit and an organic binder such as an acrylic or ethyl cellulose. To produce a conductive circuit, firing is required to sinter the glass frit and to burn away the binder.
U.S. Pat. No. 2,687,395 discloses methyl methacrylate polymers of improved electrical conductivity which are produced by dissolving a metal halide such as aluminum, zinc, or stannic halide in methyl methacrylate and subjecting the mixture to polymerization conditions.
Resin bonded electrical resistors are disclosed in U.S. Pat. No. 3,056,750. The resistor material comprises a plurality of discrete units which are dispersed in a polymerized resin. Each discrete unit comprises an aggregate of conductive particles of either carbon or metal, or both. The particles are bonded together by a solid polymerized resinous material to form the discrete unit. The metal particles can be copper, iron, aluminum, zinc, etc. However, merely mixing a copper powder with a resin results in a product which has poor conductivity because the peripherial edges of the copper powder particles are oxidized by oxygen in the air to form a coating of copper oxide.
U.S. Pat. No. 3,867,315 discloses conductive resinous compositions which contain a copper metal-containing solid and a metallic salt such as zinc chloride. The conductive compositions are prepared by incorporating the metal particulate and metal salt into a polymer by conventional polymer blending techniques. Conductivity is achieved by heating the blended composition. The space between the copper metal particles is occupied by the metal salt to impart conductivity to the resin. Inclusion of a stabilizing agent is not disclosed.
U.S. Pat. No. 3,983,075 teaches a conductive, cured epoxy resin which is filled with copper flakes. Treatment of the copper flakes to remove surface deposits prior to admixture with the resin is taught as being essential. Immediate use of the copper flakes and immediate curing of the resin is needed to avoid degradation of the flakes. Also, it has been found that a cured product prepared, for example, by dissolving copper oxide with an acid to form a copper oxide-free copper powder and mixing the copper powder with a resin followed by curing, cannot retain its conductivity in air for more than about one month.
U.S. Pat. No. 3,014,818 discloses the production of electrically conductive polymeric articles by admixing a polymer with a reducible salt of copper, for example, and forming the admixture into a shaped article. The article is then treated to reduce the salt to the free metal. The free metal particles, it is disclosed, are dispersed throughout the polymeric material. The shaped articles are then plated by using a plating solution consisting essentially of an aqueous alkaline solution of a nickel or cobalt salt, a borohydride, and an agent which forms a complex with nickel or cobalt. The complexing agent keeps the nickel and cobalt in solution. Exemplary of the complexing agents are ammonia and organic complex-forming agents which contain amino groups, imino groups, carboxy groups or hydroxy groups. The shaped articles, it is disclosed, can also be plated with copper.
U.S. Pat. No. 4,122,143 discloses the production of conductive cured products by reacting a copper compound which is in the cuprous and/or cupric state with a reducing substance capable of reducing the copper compound to metallic copper. The reaction occurs in the presence of both a metallic copper powder and a resinous curable component on a substrate. The copper compound is reduced to metallic copper which precipitates on the metallic copper powder to link the particles of the metallic copper powder to form a conductive connected unit of the metallic copper powder. The resinous curable component is cure moulded during the reduction of the copper compound to integrate the connected unit and the resinous curable component into the conductive cured product.
When organic or inorganic phosphorous acid type compounds such as mono or diesters of phosphorous acid are used as a reducing agent in the process of U.S. Pat. No. 4,122,143, cured resins having a superior initial conductivity are obtained. The phosphorous-containing substances, it is disclosed, have an antirust effect on the metallic copper powder. However, applicants have found that when the cured resin of U.S. Pat. No. 4,122,143 is kept under high temperature and high humidity for a long period of time, a substance having the appearance of a green rust grew on the surface of the cured resin. Also, the conductivity of the cured resin decreased gradually under the high temperature and high humidity conditions.
Incorporation of known additives at the time of curing the resins is disclosed in U.S. Pat. No. 4,122,143. Among the additives disclosed are stabilizers such as hydroquinone, p-benzoquinone, phenothiazine, t-butyl catechol, and the like, chelating agents such as acetylacetone, ethylenediaminetetraacetic acid, triethanolamine, nitrilotriacetic acid, phytic acid, and the like, acidity modifiers such as amines, alkali metal hydroxides, carboxylic acids, acid salts, and the like. While the patent discloses the use of a compound having a dihydroxy benzene ring as a stabilizer, its use as a reducing agent to reduce the copper compound to metallic copper is not dislcosed. Additionally, the use of chelating agents to protect against the formation of a skin on the surface of the resinous composition for producing conductive cured products is not disclosed.
According to the present invention, there is provided a resinous composition for producing conductive cured resinous products which products have a superior initial conductivity. The cured products do not substantially generate a green rust-like substance under high temperature and high humidity storage conditions. Also, even under the latter conditions the conductivity of the cured products decreases only slightly and their resistance value does not change substantially. These unexpected results are obtained without the need for phosphorous-containing reducing substances which have a P-OH group as disclosed in U.S. Pat. No. 4,122,143.
The present invention also provides a resinous composition for producing conductive cured products which composition does not exhibit a skin forming phenomenon on its surface during storage for at least three months.